1. Field of the Invention
The present invention generally relates to amplifiers and devices having a bias circuit and conductance control methods, and more particularly to an amplifier having a bias circuit for transconductance compensation, a device having such a bias circuit, and a transconductance control method for controlling a transconductance of a transistor within such an amplifier. The present invention also relates to a transconductance control method for controlling a transconductance of a transistor within a device, such as a limiter and a mixer, that is provided with a bias circuit.
2. Description of the Related Art
In a radio communication equipment, for example, a transmission power amplifier has a large power consumption because of the need to obtain a large power output, and the transmission power amplifier is provided in a final stage for outputting the transmission wave. For this reason, there are demands for the transmission power amplifier to have a small power consumption and a small inconsistency in the output characteristic.
FIG. 1 is a circuit diagram showing an example of a conventional amplifier. An amplifier 1 includes a transconductance (Gm) compensating bias circuit 2, and an amplifier circuit 3 that are connected as shown in FIG. 1. The amplifier circuit 3 includes resistors R1 and R2, a capacitor C1 and a transistor Tr1. The Gm compensating bias circuit 2 is provided to maintain the transconductance (Gm) of the transistor Tr1 constant, and compensates a gate voltage of the transistor Tr1 by outputting a bias voltage Vg. In FIG. 1, “Vcc” denotes a power supply voltage, “in” denotes an input signal, and “out” denotes an output signal.
FIG. 2 is a diagram showing a relationship of the transconductance Gm (arbitrary units) of the amplifier 1 shown in FIG. 1 and the bias voltage Vg (arbitrary units) output by the Gm compensating bias circuit 2 of the amplifier 1. Due to inconsistencies in the manufacturing processes when the transistors Tr1 and the like are manufactured, the fluctuation in the power supply voltage, the temperature change and the like, the transconductances Gm of the individual transistors Tr1 become inconsistent as may be seen from characteristics A, B and C shown in FIG. 2. For this reason, because the characteristic C has the lowest transconductance Gm of the characteristics A, B and C, a region of the characteristic C having a large transconductance Gm must be set as a using region UR1 of the transistors Tr1, and the transconductance Gm in the using region UR1 is limited to a low value.
For example, a Japanese Laid-Open Patent Application No. 2000-91861 proposes a gain control method for increasing IIP3 during operation at a low gain, and a Japanese Laid-Open Patent Application No. 2003-188653 proposes a bias circuit for an amplifier circuit having a small temperature dependency.
In the conventional amplifier 1, if an attempt is made to suppress the inconsistencies in the characteristics of the transistors Tr1, the transconductance Gm in the using region UR1 of the transistors Tr1 is limited to a low value, and there was a problem in that it is difficult to realize a high performance of the amplifier 1. On the other hand, if the using region UR1 of the transistors Tr1 is set so that the transconductance Gm in the using region UR1 becomes high in order to realize the high performance of the amplifier 1, there was a problem in that the inconsistencies in the performances of the amplifiers 1 become large due to the inconsistencies in the characteristics of the individual transistors Tr1. In other words, realizing the amplifier 1 having the high performance and realizing the amplifier 1 with the suppressed performance inconsistency are in a tradeoff relationship.
In addition, in the case of the amplifiers that produce large power outputs, there are cases where it is difficult to sufficiently suppress the inconsistencies in the performances, and there are also cases where it is inevitable to employ a design that sacrifices the linearity of the amplifier output.
Furthermore, problems similar to those described above with respect to the amplifier were also encountered in devices other than the amplifiers, such as limiters and mixers.